As shown in U.S. Pat. No. 4,113,268 dated Sept. 12, 1978, a sealing assembly for a butterfly valve is illustrated in which a soft primary seal is positioned between a pair of spaced metal secondary seals. The secondary metal seals are held out of contact with the seating surface of the butterfly disc by the soft seal under normal operating conditions. Only when the soft primary seal is destroyed do the secondary metal seals engage the disc in sealing relation. A separate resilient backing ring urges the primary soft seal and the secondary metal seals inwardly toward the seating surface of the disc.
U.S. Pat. No. 4,162,782, issued July 31, 1979 to Ronald D. Wilkins and filed Apr. 10, 1978 for "Seal Assembly for Butterfly Valve" discloses a seal assembly mounted in an annular groove about the flow passage. The seal assembly includes a metallic body having a pair of outer legs seated in the bottom of the groove and a pair of inner legs contacting the outer periphery of the valve disc. A soft seal is positioned between the inner legs and the inner ends of the inner legs contact the valve disc to provide metal sealing surfaces. Upon contact with the disc, the inner legs are urged radially outwardly. However, the inner legs do not have a large radial movement and a substantial frictional force is provided by contact with the inner legs upon radially outward movement which increases the operating torque for moving the valve between open and closed positions. With certain types of metal finishes or coatings on the valve disc, some scratching of the sealing surface on the valve disc might occur with a high frictional contact, particularly if the seal has a so-called knife edge metal contact surface.